, Siemens AG, Mülheim/Ruhr, Germany Extended requirements on turbogenerators due to changed operational regimes siemens.com
Table of Content Evaluation of current operation regimes Extended requirements on turbogenerators Fast active & reactive load changes Load ramps Under-excitation Over-voltage Possible Solutions and Mitigations Conclusions Page 2
Evaluation of Operation Regimes Air Cooled Generators, 300 MVA Class Worldwide disposition of the generators in the 50 Hz market Detailed evaluation from commissioning up to 2014 Strong dependency on renewable share and grid connection Increasingly frequent permanent load fluctuations One specific generator reactive power Relative frequency of operation point (P, Q) [%] Summarized load capability diagram of the investigated > generator fleet with relative frequency of operation points High number of start-stop cycles in % of all units Operation in whole released capability range High active share power of reactive power for grid stabilization Full use of under-excitation capability because of capacitive grid demands Variable Increased and specific demand operation on highly stresses flexible for load operation generators of conventional of the same power class plants Generator Nr. Distribution of reactive power operation of all 33 units over-excitation under-excitation 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 mean 20% mean 80% Page 3
Extended requirements on turbo-generators Overview Increased requirements Fast active & reactive load changes Load ramps up to 24 % of rated MW / min Under-excitation Over-voltage Physical / technical challenges High thermomechanical tension at windings Thermal cycling High magnetic flux in end region High magnetic flux density Expected strain in respect to cooling method Indirectly Generator components cooled Directly cooled Main bushings of stator winding Mid Low Carbon brushes and slip rings of static excitation Low Low Stator core end zones (stepped teeth) Mid Low Stator winding, especially overhangs High Low Rotor winding, especially end-windings covered by retaining rings High Mid Complete stator winding High Low Complete rotor winding High Low End teeth, press finger, press plate High Mid Stator winding in stepped core area High Low Stacking beams at stator core back High High Rotor winding High Mid Stator core insulation Low Low Page 4
Fast active & reactive load changes, ramps Thermo-mechanical stress on the stator winding insulation system ΔP in MW or ΔQ in Mvar alteration of stator current ΔI RST alteration of stator winding losses (ΔP V ~ ΔI RST2 ) change of stator winding temperature (ΔT ~ ΔP V ) Physical effect: Thermo mechanical stresses on the insulation system due to Different thermal expansion coefficients of copper, insulation and steel Different temperature levels Positive load change + ΔP, ΔQ Negative load change - ΔP, ΔQ Stator winding bar T T Copper conductor Insulation Generic cyclic thermo-mechanical loading Insulation Copper condctor Rel. occurrence of current ramp ΔI RST /Δt [%] Generator Nr.1: High amount of steep current ramps Steepness of current ramp Page 5
Fast active & reactive load changes, ramps Detailed evaluation of thermo-mechanical stress on the stator winding insulation system stator core Individual modeling of stator bar design including copper conductor, insulation sleeve and interface Challenging effort of large end winding geometry compared to thin/tiny insulation sleeve geometry Detailed knowledge about temperature dependent mechanical properties of insulation materials Validation by strain and deflection measurements in operation behaviour, continous calibration of design tools stator core High thermo-mechanical stress at first bend Detailed assessment of highly stresses areas during load transients Page 6
Fast active & reactive load changes, ramps Indirect cooled stator winding, inner/outer corona protection Design characteristics of GVPI insulation system Copper strands Verification of designed shear plane (ICP) by detailed material tests Designed shear planes (ICP/OCP) reduce thermo mechanical stresses on groundwall insulation Page 7
Over-Voltage / Under-excitation / Start-stop cycles Stator Core, Generator Rotor Stator Core Risk of magnetic over-fluxing @ increased voltage and frequency fluctuation Capability to maintain leakage flux and circulating currents at the back of the core Under-excitation impact on end zone Taken from: IEEE-PES- 2012_WG8-Panelpaper_Grid Code Impact to Machine-design Generator Rotor Mechanical integrity covered by extended analysis: LCF (start-stop cycles) Wider grid frequency range (natural frequencies) Transient events Fast and frequent thermo cycling at the rotor winding: Equal temperature distribution in the winding, no significant hot spots Winding design allows fast thermal expansion and contraction of copper Insulation materials are designed to sustain cyclic stresses for long term operation All requirements must be considered in the design work Page 8
Possible Solutions and Mitigations Fast active & reactive load changes, ramps Power S, Temperature T Variation of stator winding Temperature with conventional cooling system Stator winding temperature, e.g. slot RTD Generator Cooler Generator Load Time t Simple Cooling Water System without active regulation Conventional static generator cooling system results in high variation gradient of winding temperature and thermo-mechanical stresses Page 9
Fast active & reactive load changes, ramps Enhanced temperature control system Less variation of stator winding temperature with load change Power S, Temperature T Stator winding temperature (slot RTD) with an active operating control loop Schematic diagram of active controlled generator cooling system Generator Load Smoothing of temperature variation higher T level Controller Time t Process variable input e.g. slot RTD, warm gas Dynamic control of cooling gas temperature with new water cooler system Reduced thermo-mechanical stress in winding materials Page 10
Possible Solutions and Mitigations Under-excitation / Radial flux effect Flat stator core end region reduces flux heating in copper strands in over-excited operation mode (lagging p.f.) optimal design optimal design meets future extended requirements optimal design r-axis High magnetic flux in stepped core end stator core rotor top coil Indirectly cooled stator winding requires a compromise to stay within temperature limits of stator coil stepped iron Steep stator core end region reduces heating in stepped iron in under-excited operation mode (leading p.f.) Best design to meet extended requirements: Directly water cooled stator winding design Steep stator core end region Page 11
Possible Solutions and Mitigations Product life cycle philosophy, future targets Robust Product Design Engineer toolbox Validation process Fleet experience Power plant process Optimization Improved process of plants Monitoring & Diagnostics Continous data assessment Condition & Fleet experience based maintenance concept Flexible inspection schedule Specific retrofit recommendation Probability to failure XXX XXX XXX XXX XXX Life cycle assessment Aging of components Risk evaluation Dynamic counter Low Contingency risk Rotor Stator Winding High Page 12
Condition Based Maintenance Future Goal Example stator winding 1 Kind of loading Measurement Analysis Aging effect Thermo-mechanical loading Dynamic vibration load Stator current, Cold gas temp Static forces, strains Fiber optic vibration Dynamic forces measurement at end windings Debonding effects, loosening support structure Loosening end winding structure Electrical field load Partial discharge Pattern comparison Degradation HV-insulation Transients during electrical fault operation All electrical data Short circuit forces, strains Coil insulation at core end High thermo-mechanical load at slot exit 1 Low Risk assessment stator winding Contingency risk High Stator winding Page 13
Condition Based Maintenance Future Goal Example stator winding Kind of stressing Measurement Analysis Aging effect Thermo-mechanical stress Stator current, Cold gas temp Static forces, strains Cracks in the HVinsulation material 2 Dynamic vibration load Fiber optic vibration measurement of end windings Dynamic forces Loosening end winding structure Electrical field load Partial discharge Pattern comparison Transients during electrical fault operation All electrical data Short circuit forces Degradation HV-insulation Coil insulation at core end Harmonic Stator End Winding Analysis 2 Low Risk assessment stator winding Contingency risk High Stator winding Page 14
Condition Based Maintenance Future Goal Example stator winding Kind of stressing Measurement Analysis Aging effect 3 Thermo-mechanical stress Dynamic vibration load Stator current, Cold gas temp Static forces, strains Fiber optic vibration measurement of end windings Dynamic forces Partial discharge tanδ 0 values, Δtanδ 0 rise Pattern comparison Transients during electrical fault operation All electrical data Short circuit forces Partial discharge measurement of HV winding insulation Cracks in the HVinsulation material Loosening end winding structure Degradation HV-insulation and grading system Coil insulation at core end 3 aged Risk assessment stator winding new Low Contingency risk High Stator winding Page 15
Condition Based Maintenance Future Goal Example stator winding Kind of stressing Measurement Analysis Aging effect Thermo-mechanical stress Dynamic vibration load Stator current, Cold gas temp Static forces, strains Fiber optic vibration measurement of end windings Dynamic forces Cracks in the HVinsulation material Loosening end winding structure 4 Electrical field load Partial discharge Pattern comparison Transients during electrical fault operation All electrical data Short circuit forces Degradation HV-insulation Coil insulation at core end Transient Analysis of Fault conditions 4 Risk assessment stator winding Low Contingency risk High Stator winding Page 16
Extended requirements on turbo-generators Conclusions New flexible grid demand has impact on whole system generator with different amount of wear and aging at individual components Changed requirements and remaining uncertainty for future increase of flexibility must be considered in the current generator development programs Thermo-mechanical stresses on generator components require enhanced load dependent cooling technology, particularly at the stator winding Based on new EOH calculation with load change factor (VGB R 167 2010) condition based maintenance is needed new economic maintenance strategies for the generator Thank you for your Attention! Page 17
Extended requirements on turbo-generators due to changed operational regimes Contact page Phone: +49 (208) 456 8222 Mobile: +49 (174) 1534169 E-mail: matthias.baca@siemens.com Rheinstr. 100 45478 Mülheim an der Ruhr Germany Page 18
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